PERIODATE OXIDATIONOF
Sept,, 1951
577OK., is represented to within an average deviation of 0.3% by the equation HT
- Hzos.ts=
29.56T
+ 6.11 x
1062'-1
-
10,863
In Table 111 also are values for the liquid a t the melting and boiling points, 577 and 592OK., respectively, and for the gas (in terms of 0.5 mole Fed&) at the b o d q point. These values were ob-
[CONTRIBUTION FROM
THE
THE
FORMYLESTERSOF STARCH
4185
tained by supplementing the present data with the heat of fusion and vaporization values from the N. B. S. table^.^ (4) National Bureau of Standards, Tables of Selected Values of Chemical Thermodynamic Properties, Ser. 11, Table 47-2 (Sept. 30. 1949).
BERKELEY 4, CALIFORNIA
RECEIVED U C H
14, 1951
CHEMICAL LABORATORIES, GEXERAL MILLS, Isc.]
Periodate Oxidation of the Formyl Esters of Starch1 BY OWENA. MOE,SIDNEY E. MILLERAND MARJORIE IWEN BUCKLEY The formyl esters and the 2-cyanoethyl ethers of diverse starches have been subjected to periodate oxidations and it has been found that the consumption of oxidant is dependent on a number of variants. The results of the current study show that the selectivity of the periodate is dependent on the concentration of the oxidant and the PH of the media.
In connection with some of the periodate oxidation studies2 underway in this Laboratory, it beecame of interest to investigate the oxidation of the formyl esters of starch by periodate. Previous investigators3 observed that the periodate oxidation of the formyl esters of corn starch (essentially a monoformate, degree of substitution approximately 1.1) proceeded in the same general course as that observed with the original starch although the reaction was slower for the formate ester. They found that approximately 1 mole of periodate had been consumed per mole (substituted anhydropyranose unit) of derivative after a reaction penod of one hundred twenty hours. The results of the periodate oxidation studies indicated that the formyl group was located primarily in the Ca-position. In view of our previous observations2a relative to the importance of the fiH of the oxidation medium and the temperature a t which the oxidation was carried out, it was considered possible that considerable hydrolysis accompanied oxidations of this type when the proper conditions were not imposed. This assumption was verified by the experimental evidence presented in this paper. In agreement with Gottlieb, Caldwell and HixonJa it was found that formyl esters of starch were slightly in excess of monoformates. The formyl content of the esters was usually within the range 16.3-16.8%. When the oxidations involving the starch formates were carried out in buffered (0.05 M)2a media (pH 4.2-4.3) approximately 0.6-0.7 mole of oxidant had been consumed per mole of derivative after a 96-hour period as shown in Table I. This proved to be so with the formates of either cereal or tuber starches. However, when the oxidations with periodate were carried out in unbuffered media, completely different results were obtained, and the final (1) Paper No. 99, Journal Series, General Mills, Inc., Research Dept. (2) (a) 0. A. Moe, S. E. Miller and Marjorie R. Iwen, THISJOURNAL, 60,2621 (1947); (b) H. WittcoB. 0. A. MOLand M. H. Iwen, ibid., TO, 742 (1948). (8) D. Gettlieb, e. G. Caldwell and R. M. Hixon, ibid., 68, 3342
(1040).
TABLEI FORMATES IN BUFFERED (0.05 AI) MEDIA,19"
PERIODATE OXIDATIONS OF STARCH
Source
Formyl, %
Degree of substitution
Wheat
16.8
1.13
Potato
16.5
1.10
Corn
16.8
1.13
Reaction time in hours
24 48 96 24 48 96 24 48 96
Mole periodate consumed/mole derivative
0.58 .60 .61 -60 .62 .66 .60 .66
.69
values were in all cases at least twice as high as those obtained when the oxidations were carried out in buffered media. The results of the oxidations in unbuffered media are presented in Table 11. TABLE I1 PERIODATE OXIDATIONSOF STARCH FORMATES IN UNBUFFERED MEDIA,23" Source
Degree substitution
Wheat
1.15
Molar concentration of
HI04
0.5
-05 Potato
1.10
.5 .05
Reaction Moles periodate time in consumed/mole hours derivative
24 96 24 48 96 24 96 24 48 96
1.08 1.64 0.99 1.19 1.32 1.19 1.77 1.09 1.21 1.34
The molar concentration of the aqueous periodic acid employed appeared to have an appreciable effect and substantially higher values were obtained as the molar concentration was increased. From the tables, it becomes apparent that the oxidations carried out in buffered media were substantially complete after 24 hours and an increase of only 0.06 0.03 mole of periodate per mole of derivative occurred during the next 72-hour period. Howeves, when the oxidations were camed out in unbuffered media, the increase
*
4186
OWEN
A. MOE,SIDNEY E. MILLERAND MARJORIE IWEN BUCKLEY
in periodate consumption during the next 72-hour period ranged from approximately 0.3 mole to approximately 0.6 mole per mole of derivative depending on the molar concentration of aqueous periodic acid employed. In view of the above results, it was desirable to study the periodate oxidation of a different water insoluble starch derivative, preferably a derivative which would not be so prone to undergo hydrolytic cleavage under the conditions employed for the oxidation studies. The 2-cyanoethyl ether of starch was considered suitable for this purpose. It was prepared by the reaction between wheat starch and acrylonitrile in the presence of an alkaline catalyst essentially as described by Bock and H ~ u k .The ~ nitrogen analysis indicated that the 2-cyanoethyl ether derivative possessed a degree of substitution of approximately 1.1. Hence, it was comparable to the starch formates which were studied. Accordingly this 2-cyanoethyl ether of wheat starch was subjected to oxidation by periodate in both buffered and unbuffered media and the results obtained are given in Table 111. TABLE I11 PERIODATE OXIDATIONS O F THE 2-CPANOETHYL ETHERO F U’HEAT STARCH Periodate used
Buffered (0.05 M ) Unbuffered (0.5 iM)
Reaction time in hours
48 96 45
96
Moles periodate consumed/ mole derivative
0.56 58 .59 66
% N of oxidized product
7 03 6.41
The oxidations were essentially complete after 48 hours. A greater “drift” in the consumption
of the oxidant was observed when the oxidation was carried out in 0.5 molar unbuffered periodic acid. However, it is readily apparent that the 2-cyanoethyl ethers are much more resistant to hydrolysis than are the formyl esters under similar oxidation conditions. The find values (approximately 0.6 mole of oxidant consumed per mole of derivative) are indeed comparable to the values obtained by the periodate oxidations of the formyl esters of starch in buffered media. Hence it appears that the formyl groups and the 2-cyanoethyl groups are distributed in a similar manner in the starch molecule and it appears that the formyl groups are substituted predominantly a t the C6-position3although not exclusively so. The results of the present investigation clearly indicate that the concentration of the oxidant and the pH of the media (as well as the temperature? are very important factors to consider in (4) Bock and Houk, U. S. Patent 2,316,129
Vol. 73
the periodate oxidations of complex molecules. The selectivity of periodic acid oxidations is dependent on these considerations.
Experimental Preparation of Starch Formates.-The formyl esters of starch were prepared essentially as described by Gottlieb, Caldwell and Hixon3 except that the products were precipitated in acetone, mixed thoroughly with acetone in a Waring blendor, collected, washed with acetone and dried extensively in ouuo. The formyl esters were obtained as white powdery products. Anal. Calcd. for starch monoformate: ClrHloOe, 15.26. Found: for wheat, 16.8; for potato, 16.5; for corn, 16.8. Periodate Oxidation of the Formyl Esters of Starch. A. Buffered Media.-The buffered oxidation reactions were carried out using 0.5 g. (dry basis) of the formyl esters and 350 cc. of an 0.05 M solution of periodate.” The reaction temperature was maintained at 10’ and the formyl esters were kept in a finely divided state. The titrations of aliquot portions of the periodate solution were carried out at various time intervals according to the method of Fleury and Lange6 as modified by Grangaard, Gladding and Purves.6 The results obtained from the buffered oxidations at 19’ are summarized in Table I. B. U n b d e r e d Media.-The unbuffered oxidations were carried out using 0.5 g. (dry basis) of the formates and either 350 cc. of 0.05 M or 60 cc. of 0.5 M aqueous periodic acid. When the unbuffered oxidations were carried out in 0.5 M aqueous periodic acid, it was observed that the formyl esters of starch had gelatinized to an appreciable extent. The titrations were carried out as previously noted and the results from the experiments using 0.05 and 0.5 M aqueous periodic acid are summarized in Table 11. Preparation of the 2-Cyanoethyl Ether of Wheat Starch.-It was desirable to prepare a cyanoethyl ether of wheat starch possessing approximately 1.1 cyanoethyl groups per anhydropyranose unit. This was achieved in the following manner: Wheat starch (33 9.) was dispersed in 300 cc. of water. Aqueous sodium hydroxide (10 g. of a 20% solution) was added and the resulting reaction was stirred vigorously until a uniform dispersion was achieved. Acrylonitrile (31.8 8.) was added and the resulting reaction mixture was stirred for a three-hour period at room temperature. After standing at room temperature for an additional three-hour period, the product was precipitated as a gummy mass by pouring into an excess of methanol. This gummy mass was mixed vigorously with fresh methanol in a Waring blendor and it gradually hardened. Finally, the 2-cyanoethyl ether derivative was ground under methanol to yield a fine powder, collected, washed with methanol and dried in vacuo. Anal. Calcd. for the mono-2-cyanoethyl ether (CsHs0,N): N, 6.67. Found: N, 6.94. Periodate Oxidation of the 2-Cyanoethyl Ether Derivative of the Wheat Starch.-The buffered and unbuffered oxidations were carried out essentially as described for the corresponding oxidations of the formyl esters of starch. The insoluble oxidation products were collected, washed extensively with water and methanol and dried in vacuo. These insoluble cleavage products were analyzed. Anal. Found: for buffered oxidation product: N, 7.03. Found: for unbuffered oxidation product: N, 6.41. MIIU”. MINNEAPOLIS,
RECEIVEDMARCH 29, 1951
(5) P. Fleury and J Lange, J . pharm. Chem., [SI 17, 107, 196 (1933). ( 6 ) D. H. Grangaard, E. K. Gladding and C. B. Purves, Paper Trade J , 116, 41 (19.12).
